The present invention relates generally to shift systems for power transfer devices and, more particularly, to a hydraulically-actuated shift system for a part-time four-wheel drive transfer case.
With the advent of increased consumer popularity in four-wheel drive passenger cars and sport/utility vehicles, power transfer devices are more frequently being incorporated in vehicular driveline applications. As is known, part-time transfer cases are used in four-wheel drive vehicles for selectively directing power to the non-driven wheels upon shifting from the two-wheel drive mode to the four-wheel drive mode. More particularly, most conventional transfer cases use a transmission type synchronizer clutch arrangement incorporated with a shift fork mechanism to provide "shift-on-the-fly" two-wheel drive to four-wheel drive mode shifting. When shifting the transfer case between the two-wheel and four-wheel drive modes, selective movement of a shift lever by the vehicle operator causes corresponding movement of the sniff fork mechanism between positions defining the two-wheel and four-wheel modes. Alternatively, the shift fork mechanism may be remotely actuated, as in an electronically-controlled shift system equipped with an electric motor actuator arrangement. In either case, movement of the shift fork mechanism toward the four-wheel drive position acts to energize the synchronizer clutch apparatus for shifting a clutch sleeve into engagement with the external spline of a silent chain carrier. In this manner, the transfer case is "locked-up" in the four-wheel drive mode for delivering power to the vehicle's front and rear wheels.
A common problem associated with many conventional part-time transfer cases is that it is often difficult to shift into the four-wheel drive mode. In manually-actuated part-time transfer cases, the vehicle operator is required to exert a large input force on the shift lever to physically overcome the shift resistance associated with shifting into the four-wheel drive mode. Similarly, in electrically-actuated shift systems the electric motor must continue to provide the large input force during an extended shift period which, in some instances, may overload the electric motor. In other instances, the electric motor may generate an excessive amount of shifting force for causing the shifting process to be noticeably harsh and abrupt.